Length is not the only deciding factor in whether or not a game gets accepted on the site. While it is a big factor, you could create a game with an hour of gameplay and still not have it get accepted if the gameplay itself is overly simplistic. Don’t design a game with the sole purpose of getting accepted onto the site - design what you want to (and what you think will be enjoyable to play) and then go from there.
[quote=Timmiluvs]Don’t design a game with the soul purpose of getting accepted onto the site[/quote]
1000x this. This massively putting the cart before the horse (you're thinking about creating a speedrunning leaderboard... for people speedrunning a game... that doesn't even exist yet...) in a way that will almost certainly be detrimental to the game you actually want to make, but also, frankly, this is exactly the kind of thing we're not looking to add to the site.
In any case, to answer the original question: [quote=rm]How long do you guys think the game should take to finish for it to be accepted to speedrun?[/quote]
Since games are considered on a case-by-case basis there isn't really a hard length requirement, but generally speaking, roughly 5 minutes of casual play is usually the absolute bare minimum that we'll seriously consider.